THE VOLCANIC ROCKS OF ICELAND. 



93 



have already described above, has exercised precisely the same 

 influence upon the surrounding rock, which has thus for some con- 

 siderable distance round been converted into a pitchy black mass, 

 nearly resembling obsidian, sometimes dull and sometimes almost 

 brilliant in appearance, containing calcareous spar and zeolites 

 intimately disseminated throughout, and which, to judge from its 

 zeolitic admixture and frequently still recognizable imbedded 

 earthy fragments of pyroxenic rock, is the product of the meta- 

 morphosis of a palagonitic tuff. Analysis gave the following 

 mean composition for this rock : — 



The absence of pyrites and gypsum proves that it was car- 

 bonic acid alone which caused the metamorphism in the moist- 

 ened rock. Here likewise, if the per-centage of lime contained 

 in the original rock is restored from the disseminated calcareous 

 spar, we obtain as the composition of the anhydrous rock, — 



111. 



Sihca 50-25 



Alumina 12*54 



Protoxide of iron . . 16*15 



Lime 11-09 



Magnesia .... 7*59 



Soda 2*04 



Potash 0-34 



100-00 



By this means we again obtain a composition for the original 

 rock, which is almost exactly the same as that of the normal py- 

 roxenic substance, and indeed with the oxygen relation 3 : 1-81, 



